Oxetanes are strained, reactive heterocyclic compounds that undergo facile cationic ring-opening polymerization and this topic has been the subject of two review articles (Saegusa, T. J. Macromol. Sci. Chem. 1972, 6, 997-1026; Penczek, S.; Kubisa, P.; Matyjaszewski, K. Adv. Polym. Sci. 1985, 68/69, 66-77). Some of the seminal studies of the cationic ring-opening polymerizations of these compounds were performed by Kops et al. (Kops, J.; Spanggaard, H. Cationic Polymerization and Related Processes, Goethals, E. J. editor, Academic Press, San Diego 1984, 227-236; Kops, J.; Hvilsted, S.; Spanggaard, H. Macromolecules 1982, 15, 1200-1201; Kops, J.; Spanggaard, H. Macromolecules 1982, 15, 1225-1231; Kops, J.; Spanggaard, H. Cationic Polymerization and Related Processes, Goethals, E. J. editor, Academic Press, 1984, 220-236), Dreyfus and Dreyfus (Dreyfus, P.; Dreyfus, M. P. Polym. J. 1976, 8, 81-87) and by Goethals et al. (Goethals, E. J. Adv. Polym. Sci. 1977, 23, 101; Bucquoye, M.; Goethals, E. J. Makromol. Chem. 1978, 179, 1681-1688). Oxetanes possess a high degree of ring strain (107 kJ/mol) that is only slightly less than epoxides (114 kJ/mol) (Pell, A. S.; Pilcher, G. Trans. Faraday Soc. 1965, 61, 71-77). On the other hand, oxetanes are considerably more basic (pKa=2.0) than epoxides (pKa=3.7) (Searles, S.; Tamres, M.; Lippincott, E. R. J. Am. Chem. Soc. 1953, 75, 2775-2778; Arnett, E. M.; Progress in Physical Organic Chemistry, Interscience, New York, 1967, 7, 243). These two factors offset one another and lead to the prediction that both classes of monomers should have similar reactivity in cationic ring-opening polymerizations. Despite this conclusion, until recently oxetanes have received comparatively little attention from both academic and industrial researchers while research publications on epoxide monomers, oligomers and resins abound.
This is slowly beginning to change due to several significant factors. At the present time, epoxide resins based on bisphenol-A and epichlorohydrin have received a great deal of negative press regarding ongoing health and worker safety concerns. Bisphenol-A has been reported to be an estrogen mimic (O'Connor, J. C.; Chapin R. E. Pure Appl. Chem. 2003, 75(11-12), 2099-2123; vom Saal, F. S.; Myers, J. P. J. Am. Med. Assoc. 2008, 300(11), 1353-5), while epichlorohydrin used in the synthesis of these resins is an orally and dermatologically active toxic agent (Lawrence, W. H.; Malik, M.; Turner, J. E.; Autian, J. J. Pharmaceut. Sci. 1972, 61, 1712-1717) as well as a human mutagen and carcinogen (IARC Monographs on the Evaluation of Carcinogenic Risks in Humans, Monograph 71, 1999, 603-628). The corresponding 3,3-disubstituted oxetane monomers do not have these drawbacks and, in general, have low orders of acute and chronic toxicity and are also non-mutagenic (Sasaki, H. Photoinitiated Cationic Polymerization, Chapter 26, Belfield, K. D.; Crivello, J. V., editors, ACS Symp. Ser. 847, Am. Chem. Soc., Washington, D.C. 2003, pp. 296-305). Oxetanes with substituents at the 2 and 4 positions undergo sluggish cationic ring-opening polymerizations, while those with substituents in the 3 position are considerably more reactive.
An additional factor that accounts for the relatively slow development of oxetane technology relative to the well-entrenched epoxide chemistry appears to have been the general lack of commercial availability of analogous 3,3-disubstituted oxetane monomers and oxetane functional oligomers. However, in recent years, the development of versatile and practical synthetic methods that afford mono-, di- and multifunctional 3,3-disubstituted oxetanes in high yields has made these monomers freely available from several commercial sources.
Two of the major synthetic routes for the synthesis of 3,3-disubstituted oxetanes are based on work originating from this laboratory and they are depicted in equations 1-4 (Sasaki, H.; Crivello, J. V. J. Macromol. Sci. Pure Appl. Chem. 1992, A29(10), 915-920. 19; Crivello, J. V.; Sasaki, H. J. Macromol. Sci. Pure Appl. Chem. 1993, A30(2&3), 189-206). In 1957, Pattison (Pattison, D. B. J. Am. Chem Soc. 1957, 79, 3455-3456) showed that 1,3-propane diols could be converted to oxetanes by reaction with diethylcarbonate in the presence of potassium carbonate. Applying this reaction to trimethylolpropane, 1, as shown in equation 1 affords the cyclic carbonate, 2, as an intermediate which on heating extrudes carbon dioxide to give 3-ethyl-3-hydroxymethyloxetane (EHMO). EHMO is a key intermediate for the synthesis of a wide variety of 3,3-disubstituted oxetane-containing monomers. For example, the reaction of EHMO with α,α′-dibromo-p-xylylene under basic reaction conditions in the presence of a phase transfer catalyst (PTC) gives difunctional oxetane monomer, XDO (equation 2).

Applying the ring closure reaction shown in equation 1 to ditrimethylolpropane (bis[2,2-dihydroxymethyl)butyl]ether), 3, gives difunctional oxetane monomer DOX (equation 3).

A third method involving a dehydrohalogenation reaction is shown in equation 4 (Hirose, T.; Ito, N. U.S. Pat. No. 5,886,199 A, Mar. 23, 1999 assigned to Toagosei, Co, Ltd. Japan). Trimethylolpropane is treated with gaseous HCl in a mixture of m-xylene and acetic acid to give 1,1-bis(chloromethyl)-1-hydroxymethylpropane, 4. After isolation, 4 is dehydrochlorinated with aqueous NaOH in the presence of a quaternary ammonium salt phase-transfer catalyst to give the desired 3-chloromethyl-3-ethyloxetane, 5. This latter compound also serves as an intermediate for the preparation of a number of mono- and multifunctional oxetanes. For example, 3-ethyl-3-phenoxymethyloxetane, POX, is prepared by an SN2 reaction of phenol with 5 in the presence of a base (equation 5).

It is worth noting that the starting materials; trimethylolpropane and ditrimethylolpropane are derived by aldol chemistry from low cost readily available and biorenewable butyraldehyde and formaldehyde. This favorable environmental aspect of oxetane chemistry provides an added attractive incentive for the further development of oxetane monomers and reactive oligomers.
In addition to the oxetane monomers prepared using the three synthetic methods described above, a number of other novel oxetanes have been synthesized such as; silicon-containing oxetanes (Crivello, J. V., Sasaki, H. J. Macromol. Sci., Pure and Appl. Chem. 1993, A30(2&3), 173-187; Sangermano, M.; Bongiovanni, R., Malucelli, G., Priola, A., Olbrych, J., Harden, A., Rehnberg, N. J. Polym. Sci., Part A: Polym. Chem. 2004, 42(6), 1415-1420; Moszner, N., Voelkel, t., Stein, S., Rheinberger, V. U.S. Pat. No. 6,096,903, Sep. 4, 2001, to Ivoclar, Ag.), oxetane-functionallized novolac resins (Nishikubo, T.; Kudo, H.; Nomura, H. Polym. J. 2008, 40, 310-316), oxetane-functional polymers for use in organic light emitting diodes (OLEDs) (Müller, C. D.; Falcou, A.; Reckefuss, N.; Rojahn, M.; Wiederhirn, V.; Rudati, P.; Frohne, H.; Nuyken, O.; Becker, H.; Meerholz, K. Nature, 2003, 421, 829-833) and oxetane esters (Nuyken, O.; B hner, R.; Erdmann, C.; Macromol. Symp. 1996, 107(1), 125-138). These efforts exemplify the great versatility of oxetane chemistry and illustrate the ease with which the oxetane group can be introduced into different substrates to produce polymerizable monomers, polymers and functional oligomers (Nishikubo, T.; Kudo, H. (2007). MRS Proceedings, 2007, 1005-Q02-04 doi:10.1557/PROC-1005-Q02-04).
The major interest of this laboratory in oxetane chemistry relates to their potential use as cationically photopolymerizable monomers and reactive oligomers in such applications as coatings, adhesives and printing inks. Two especially attractive future uses for photopolymerizable oxetanes are 3D imaging applications (sometimes termed “additive manufacturing”) and dental composites. Presented in Scheme 1 is the mechanism that we have previously proposed for the photoinitiated cationic ring-opening polymerization of oxetanes using a diaryliodonium salt as the cationic photoinitiator (Crivello, J. V. Ring-Opening Polymerization, Chapter 5, Brunelle, D. J., editor, Hanser Pub., Munich, 1963, pp. 157-196). A very similar mechanism can be written for the photopolymerization of these monomers using triarylsulfonium salts as photoinitaitors. When onium salt photoinitiators are irradiated with UV light, they undergo very efficient photolysis to generate a number of reactive species that include radicals, cations and cation-radicals (equation 6). Further reaction of these species with water or other protonic species present in the reaction mixture results in the generation of the acid, HMtXn, corresponding to the anion that accompanies the diarylidonium cation in the starting salt. Typically, the anion is selected such that a very strong protonic acid is generated which serves as the initiator (equation 7) for the subsequent polymerization (equations 8 and 9) of the oxetane monomer. The overall polymerization process is complex, involving three separate steps each with its own characteristic rate constant. From the results of our previous work (Bulut, U.; Crivello, J. V. J Polym Sci Part A: Polym Chem 2005, 43, 3205-3220; Crivello, J. V.; Bulut, U. Design. Mons. Polym. 2005, 8(6), 517-531), we suggest that the rate determining step in this reaction sequence is equation 8.

Despite the similarity of their ring-strains and steric hindrance considerations and the SN2 mechanisms of polymerization, oxetanes display a very sluggish response to onium salt-induced cationic photopolymerizations than their epoxide counterparts. Investigations have shown that this apparent sluggish polymerization behavior is manifested in the characteristically long induction periods. These results have been interpreted as due to a higher energy barrier for the ring-opening of the four membered oxetane ring than for the three membered epoxide group (Sasaki, H.; Rudzinski, J. M.; Kakuchi, T. J Polym Sci Part A: Polym Chem 1995, 33, 1807-1816; Kato, H.; Sasaki, H. Photoinitiated Cationic Polymerization, Chapter 25, Belfield, K. D.; Crivello, J. V. editors, ACS Symp. Ser. 847, Am. Chem. Soc., Washington, D.C. 2003, pp. 285-295). During the induction period, we have shown that photolysis of the photoinitiator takes place and that the photogenerated acid that is produced very rapidly protonates the oxetane monomer (Crivello, J. V.: Bulut, U. Design. Mon. and Polym. 2005, 8(6), 517-531; Crivello, J. V.; Falk, B.; Zonca, Jr. M. R. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 1630-1646). However, no substantial amount of polymerization takes place at room temperature until the acid concentration reaches a certain threshold required to support a thermally induced autoaccelerated polymerization. Often, this can be observed as a highly exothermic polymerization front that moves rapidly from one part of the sample to another within the irradiation zone. Additionally, the polymerization can be triggered by heating the sample at almost any time within the induction period.
Most importantly, the presence of an extended induction period in the photopolymerization of oxetane monomers is totally incompatible with the rapid response required for many of the high speed applications in which photoinitiated cationic polymerizations are employed. Ideally for such applications, the rate of a photopolymerization should be determined by the rate of the generation of the active species. This implies that in Scheme 1, equation 1 should be the rate determining step. Consderable efforts have been expended in attempts to minimize the induction periods of oxetane monomers or to eliminate them entirely. Most of the investigations in this area have centered about the copolymerizations of oxetanes with epoxide monomers and especially with the commercially available cycloaliphatic diepoxide, 3,4-epoxycyclohexylmethyl 3′,4′-epoxycyclohexanecarboxylate (ERL) (Sasaki, H.; Rudzinski, J. M.; Kakuchi, T. J Polym Sci Part A: Polym Chem 1995, 33, 1807-1816; Sasaki, H. Toagosei, Co. Ltd. Web publication, www2.toagosei.cajp/develop/trend/No4/oxetanes.pdf; Sangermano, M.; Malucelli, G.; Bongiovanni, R. Eur. Polym. J. 2004, 40(2), 353-358; Sasaki, H.; Kuriyama, A. Proc. RadTech Asia Radiation Curing Conference, Kuala Lumpur, Malaysia, Aug. 26-24, 1999, 263-268). However, previous work in this laboratory (Crivello, J. V.; Varlemann, U. J. Polym. Sci., Part A: Polym. Chem. 1995, 33(14), 2463-2471), demonstrated that this latter diepoxide undergoes comparatively slow cationic photopolymerizations in the presence of onium salt photoinitiators. Present disclosure provides a new approach towards enhancing the reactivity of oxetane monomers and functional oligomers that is both highly effective and general in its scope.